题目内容：
回答题The crowd stirred and whispered in awe as, on the stage, the horse slowly tapped out the beat．
Everyone became tense and quiet as the number of taps neared the correct answer to the horse trainer's question．After the final tap, the horse paused, seemed to look around and stopped．The crowd went wild !
The horse's name was Clever Hans, the Educated Horse, and was featured in a vaudeville(杂耍)
act in the early 1900s, in Europe．When asked a complicated mathematical question by his owner,
Clever Hans would tap out the correct answer with his hooves．For example, if the answer was sixty-eight, Hans would tap out six with his left hoof and eight with his right hoof．Even more remarkable, the owner would leave the room after asking the question, so there could be no secret signal between owner and horse．A mere animal seemed to be accomplishing a highly technical skill of man's!
It wasn't until years later that the secret of the trick was revealed．The owner had trained Clever
Hans to respond to slight signals．The horse became so sensitive that he learned when to stop from the crowd's reaction．Members of the audience would start involuntarily, or give some unconscious signal, when Hans reached the right answer．Modern scientists now warn against the Clever Hans syndrome(综合征), whereby researchers unconsciously give clues to their animal subjects about the actions they like to see performed!
This passage is mainly about________ A．animal intelligence
B．mathematical skills
C．Clever Hans
D．unconscious behavior
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